This invention relates generally to the filed of integrated circuit packaging and more specifically to the field of heat dissipation in integrated circuit packaging.
Many electronic devices, particularly high performance devices, generate large amounts of heat as a by-product of their operation. In particular, high speed integrated circuits generate large amounts of heat in a very small area. Since the performance of integrated circuits is degraded at higher temperatures, and their reliability is similarly degraded, much effort has been expended in the field of heat dissipation in integrated circuit packaging.
Current approaches to this problem include heat sinks or heat sink/fan combinations (where the fan blows cooler air across the heat sink), liquid immersion cooling, and spray cooling printed circuit board (PCB) assemblies. Heat sinks require a thermally conductive path from the integrated circuit die to the heat sink. This thermally conductive path generally has a relatively large inherent thermal resistance, and thus limits the amount of heat that can be dissipated from the device. When a heat sink is used to dissipate energy from several devices, the devices are often not precisely co-planar, and a compensating mechanism is required to match a single planar heat removal path to multiple devices. Even very small gaps drastically increase the thermal resistance of the path from the device to the heat sink. Also, heat sinks suffer losses due to poor fin efficiency; i.e. the thermal resistance associated with the tall, narrow form factor of many heat sink fins. This limits the amount of heat that can be removed from the devices, and becomes more significant as airflow over the heat sink is increased.
When devices are immersed in a liquid coolant, completely covering the device, direct heat transfer occurs from the device to the fluid. This fluid is then generally pumped to a heat exchanger and cooled. As a result of this design, a large volume of expensive fluid (typically 3M Fluorinert(trademark)) is required. In liquid cooling, as the name suggests, no phase change occurs at the integrated circuit, so heat removal is not as effective as in techniques that utilize a phase change of the liquid coolant. Flow paths and flow rates of the liquid coolant must be carefully controlled to avoid hot spots. Also, in a computer system, large numbers of connections in piping, pumps and heat exchangers create multiple opportunities for leaks, the subsequent loss of fluid, and the subsequent loss of cooling efficiency. Electrical pumps are generally required to circulate the fluid, and are subject to failure of the motors and seals. Pump replacement generally requires draining the system and exposing the fluid path to the atmosphere. Also, the device sub-assembly must be removed from the liquid cooling system prior to any replacement or repair activity. This operation requires draining the fluid, and creates more opportunities for leaks, spillage and evaporation. This process increases system downtime. In addition, opening the liquid cooling system to the atmosphere may allow vapor to escape into the atmosphere, which may contribute to global warming or depletion of the ozone layer.
While direct spray cooling of devices on PCB assemblies adds the advantage of a phase change at the point of cooling, there are several disadvantages in a typical installation. Chemical interactions between the working fluid and the circuit board materials, solder flux residues, solder dross residues, plastic integrated circuit packages, rubber or plastic seals, and air require that every combination be chemically analyzed and tested for long-term stability. This requirement adds direct cost and increases time to market for each combination of materials. Failure to correctly assess each combination of materials can result in reliability degradation. In addition, external heat exchangers, plumbing, and pumps add cost. Multiple connections within the system create many opportunities for leaks, and the subsequent release of vapors into the atmosphere. Constant re-circulation of the working fluid across a large surface allows solder debris and other impurities to become a problem. Filters can trap the large particles, however small solids and precipitates in suspension can re-locate on the PCB assembly to form localized high-resistance shorts. Also, constant re-circulation of a particle-laden working fluid can prematurely wear down pump components to the point of pump failure. PCB materials may eventually become permeated with the working fluid, and leak small amounts of the fluid into the atmosphere. Additional fluid must be provided in the assembly to compensate for this fluid lost to low-level leakage over time. Also, the system""s electrical reliability may be compromised if the volume of the working fluid drops below a specific point.
Liquid cooling and normal spray cooling approaches to the power dissipation problem typically are high cost solutions and of high complexity as a result of discrete pumps, piping, and condenser units. A low-cost, small form factor alternative to liquid cooling and normal spray cooling systems, while still maintaining the benefits of a phase change is needed in the art. Also, since liquid cooling and normal spray cooling systems typically require the opening of a sealed liquid unit during servicing, a system where the pump can be replaced without unsealing the liquid portion of the system is needed in the art. As concerns over the release of vapors into the atmosphere and global warming increase, there is an additional need for cooling systems that minimize working fluid leakage possibilities both during normal usage and servicing.
Heat sinks and heat sink/fan combinations often are inadequate in a high thermal load environment due to losses from interface resistance. Thus, there is a need for a high performance, compact heat dissipation system.
An assembly comprising an integrated cooling system/liquid containment system/EMI shield/pump housing/heat sink is built atop a multi-chip module. The attached devices are cooled by a spray of fluid, effecting a phase change from liquid to gas at the point of evaporation. Condensing liquid accumulates at the base of the fins and is collected by a pump for redistribution. The pump is coupled to a fan blade, which in turn is operated by a motor. A seal is formed between the multi-chip module and the integrated housing. The assembly is designed such that this seal need not be broken to service the motor, thus minimizing the amount of vapors from the working fluid lost into the atmosphere. Case fins and fan blades may be arranged in a Turbocooler (Wagner, U.S. Pat. No. 5,785,116) configuration for improved efficiency.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.